This invention relates to high speed marine vehicles utilizing air cushion principles to increase performance. More particularly, this invention is for an air cushion vehicle such as a surface effect ship or hovercraft having a bow ramp and forward flexible seal provided with means for mechanically retracting the forward flexible seal to prevent its contact and damage between the vehicle and an off-loading surface.
Both the surface effect ship (SES) and hovercraft are types of high-speed marine vehicles referred to as air-cushioned vehicles (ACV). They ride on pressurized cushions of air that separate the vehicle from the surface over which it operates. This permits much lower drag and allows greater speeds to be achieved when compared to conventional displacement hulls of equivalent power. The air cushion is contained beneath the vehicle by a flexible skirt seal system which conforms to the irregular operating surface. The seal provided by this skirt reduces the flow of air escaping from the cushion. These vehicles may also utilize a swinging bow ramp for entry and exit of cargo.
In order to maintain performance in increasing sea states, the height of the skirt and total amount of skirt material must be increased to prevent contact between the vehicle and the waves to avoid creating additional drag. Also, in order to maintain proper forward cushion stability during impacts of waves, the laterally extending bag or seal at the bow must increase in diameter as well. The technology push into operation at higher sea states is causing the skirt seals of the craft to be made even larger. This direction in the design of air cushion vehicles creates a problem during landing and loading/unloading operations, as the skirt deflates and the excess bag material of the front seal comes to rest along the sides and front of the vehicle. When the bow ramp is lowered to allow transit of vehicles on or off the craft, the landing pads of the lowered bow ramp can contact and rest on the slack bow bag, resulting in significant wear and/or puncture damage. The traversing of heavy vehicles, such as tanks going on and off the craft, increase this damage rate and create possible further damage by pinching the flexible seal between the ramp and underlying land or dock surface. As the performance envelope for ACVs continues to drive toward increasing sea states and increasing skirt heights, the vulnerability of skirt damage due to ramp systems, and the corresponding costs to replace damaged skirts will increase.
FIG. 1 depicts a disadvantageous limitation of a prior art conventional air cushion vehicle 5 mounting a laterally extending forward flexible seal 6 as part of its skirt at the bow of the vehicle. The possibility of puncturing, pinching and/or excessive wearing damage of forward seal 6 (shown in its deflated or off-cushion condition) at the bow of vehicle 5 is clearly apparent during off-loading at a landing or supporting dock surface 7. A rotatable bow ramp 8 is rotated to its lowered position as viewed from starboard, and ramp landing pads 9 bear against and pinch the rubber coated or impregnated fabric material of the deflated, flattened seal 6 against supporting surface 7. The possibility of damage of the rubber coated or impregnated fabric material of seal/skirt 6 is high, particularly when heavily laden vehicles are off-loaded, and replacement of this damaged item can remove vehicle 5 from service for extended periods during refurbishment.
Some vehicles are currently equipped with skirt retraction aids utilizing elastic cords that extend across and are attached on the inside of the skirt. The elastic cords develop tension by being elongated during inflation of the skirt seal. Once the flow of air is stopped, the skirt will deflate under its own weight and the residual tension in the elastic cords causes the bow seal to fold over on itself, away from the lowering bow ramp, thus providing the necessary clearance. However, these currently deployed skirt retraction systems have limited life due to the operating environment. The constant exposure to salt and vibration of the skirt while underway plus vibration due to wave impacts degrades the material of the elastic cords over time. Another problem with the current system is that elastic materials stretch linearly as a function of displacement. As the diameter of bow skirt seals become larger and more massive, the length and required tension must increase accordingly to effectively function. Depending on the specific geometry of the bow seal, corrosion and oil resistance, elastic materials may or may not be able to satisfy the elongation verses tension requirements, particularly on the larger seals.
Thus, in accordance with this inventive concept, a need has been recognized in the state of the art for a system for reliably retracting the flexible seal at the bow of an air cushion vehicle during off-loading at the water-land interface to reduce the possibility of damage and/or undue wear of the seal.